Oldman River Basin - Water Supply Forecasts

Recorded March to September 2004 natural runoff volume in the Oldman River basin was below to much-below-average at the Oldman Dam and Lethbridge, and below average in the southern tributaries (Belly, Waterton, St.Mary Rivers). Recorded volumes ranged from 68 to 92% of average, ranking between 18th and 31st lowest on record (Table 3). The monthly natural volume forecasts produced for the Oldman River basin were within 6.5% of recorded values, on average, for the March through September runoff period.

Entering last winter, antecedent conditions were dry as August-September 2003 volumes ranked among the lowest five in 85 years of record, near or above volumes recorded during the same period in 2001. Fall 2003 soil moisture ranged from below average in the northern half of the basin to much below average in the southern half. In the northern half of the basin, snowpacks this winter were below average. However, melt occurred a month ahead of usual, resulting in average March-April runoff volumes at the Oldman Dam. With below normal precipitation into the summer and little to no snow left, March-June volumes ranked 14th lowest on record (1912-2001 data). Normal to above normal precipitation during July-September was much needed, and produced average runoff during this period. For the runoff year (March through September), volumes in 2004 were 2% lower than in 2003.

In the southern half of the basin, snowpacks this winter were below to much below average. However, melt occurred a month ahead of usual, resulting in March-April runoff volumes which were generaly below average to average, except for the Belly River basin which had above normal runoff. With dry soils and little or no snow left, March-June volumes ranked near the 15th lowest on record (except for the Belly River which ranked 26th lowest), despite near normal precipitation through the summer. Nearly twice normal precipitation during August was much needed, and produced above average runoff during the August-September period. For the runoff year (March through September), volumes in 2004 were 6 to 13% higher than in 2003.

St. Mary River at St. Mary Reservoir

Seven monthly (February to August) natural volume forecasts were produced for the St. Mary River for the March to September period. As illustrated in Figure 4, the recorded March to September 2004 natural volume for the St. Mary River (dark blue line) was below the forecasted value (brown triangles) for February's forecast, and above the remainder of the forecasts. The initial forecast was high, but was lowered after February recorded a quarter of normal precipitation in the upper basin, and zero precipitation at several locations in the lower basin. Thereafter, forecasts were fairly similar, as near normal precipitation was recorded during the March-July period. After the final forecast, heavy August precipitation caused runoff which closely followed the upper forecast limit for the August-September period, thereby raising the March-September total volume back closer to the initial February forecast. The total recorded volume was greater than the reasonable minimum and within the probable range for all of the seven forecasts.

Forecasts ranged from 0.2% to 8.7% of the recorded volume, on average within 5.5%. The best forecast was in February, and two of the forecasts were within five percent of the recorded value (February and June).

Belly River at Mountainview

The March to September 2004 recorded natural volume for the Belly River was below-average at 226,000 dam³ or 92% of average and ranks the 31st lowest on record (based on the 1912-2001 data) (Table 3). This volume is 11% higher than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the Belly River for the March to September period. As illustrated in Figure 5, the recorded March to September 2004 natural volume for the Belly River (dark blue line) was above all seven forecasted values (brown triangles). After extremely low February precipitation, the forecasts were lowered. The forecast was raised significantly after much above normal May precipitation in the basin, and stayed there as normal precipitation fell in June-July. Much above normal precipitation during the August-September period resulted in the 18th highest volume on record for this period, raising the March-September total above the forecasted values. The total recorded volume was within the probable range and greater than the reasonable minimum on all seven forecasts.

Forecasts ranged within 3.9% to 18.5% of the recorded volumes, on average within 11.4%. The best forecast was in August, and the final three forecasts were within six percent of the recorded values.

Waterton River

The March to September 2004 recorded natural volume for the Waterton River was below-average at 506,000 dam³ or 76% of average and ranks 23rd lowest on record (based on the 1912-2001 data) (Table 3). This volume is 13% higher than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the Waterton River for the March to September period. As illustrated in Figure 6, the recorded March to September 2004 natural volume for the Waterton River (dark blue line) was above the forecasted values (brown triangles) for all seven forecasts. The initial forecast was lowered after February recorded a quarter of normal precipitation. Thereafter, forecasts were fairly similar, as near normal precipitation was recorded during the March-July period. After the final forecast, heavy August precipitation caused runoff which closely followed the upper forecast limit for the August-September period, thereby raising the March-September total volume back closer to the initial February forecast. The recorded volume was within the probable range and greater than the reasonable minimum for all seven forecasts.

Forecasts ranged within 0.6% to 8.9% of the recorded volumes, within 5.9% on average. The best forecast was in February, and three of the forecasts were within six percent of the recorded values (February, June, August).

Oldman River at Brocket

The March to September 2004 recorded natural volume for the Oldman River near Brocket was below to much-below-average at 748,000 dam³ or 68% of average and ranks the 19th lowest on record (based on the 1912-2001 data) (Table 3). This volume is 2% lower than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the Oldman River near Brocket for the March to September period. As illustrated in Figure 7, the recorded March to September 2004 natural volume for the Oldman River near Brocket (dark blue line) was below the forecasted values (brown triangles) for the first five forecasts, and above the forecasted values for the final two forecasts. Forecasted volumes steadily decreased over time as below normal precipitation fell during the March-July period. Twice the normal precipitation during August, followed by generally normal precipitation in September, resulted in the 16th highest inflow volume on record for this two month period. The total March-September recorded volume was within the probable range on six of seven forecasts (all but August) and greater than the reasonable minimum on all seven forecasts.

Forecasts ranged within 2.3% to 15.1% of the recorded volume, on average within 7.5%. The best forecast was in July and three of the forecasts were within six percent of the recorded value (May, June, July).

Oldman River at Lethbridge

The March to September 2004 recorded natural volume for the Oldman River at Lethbridge was below to much-below-average at 2,070,000 dam³ or 70% of average and ranks the 18th lowest on record (based on the 1912-2001 data) (Table 3). This volume is 6% higher than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the Oldman River at Lethbridge for the March to September period. As illustrated in Figure 8, the recorded March to September 2004 natural volume for the Oldman River near Brocket (dark blue line) was above the forecasted values (brown triangles) during May, July and August. The initial forecast, in February, was high but was lowered after February precipitation was much below normal. Thereafter, forecasts generally fell slightly each month as below normal March-July precipitation in the northern half of the basin outweighed the near normal precipitation which fell in the southern half. The June 1 forecast was an exception, as it rose slightly due to much above normal May precipitation in the Belly River basin. The recorded March-September volume total was within the probable range and greater than the reasonable minimum on all of the seven forecasts.

Forecasts ranged within 0.3% to 7.6% of the recorded volumes, within 2.4% on average. The most accurate forecast was in May and the final six forecasts were within three percent of the recorded values.

Despite wet conditions entering last winter, initial forecasts were low due to the lack of snow accumulation. Forecasts increased in April and May as a result of storms in the basin during March and April. The forecasts decreased steadily through the summer with summer precipitation being below-normal to much-below-normal. However, the recorded volume ended up outside the forecast probable range and below the reasonable minimum during July and August as the July through September recorded volume was third lowest on record, similar to the volume recorded during the same period in 2001.